1. Field of the Invention
The present invention relates to a motor speed controller for controlling rotational speed on the basis of a pulse signal in which the pulse period is inversely proportional to the rotational speed of the motor.
2. Description of the Related Art
The speed of a motor can be controlled by obtaining from a motor module a pulse signal PI in which the period varies in accordance with the rotational speed ω of the motor. A motor speed controller measures the period τP of the pulses appearing in a pulse signal, detects the rotational speed ω of the motor from that period, and drives a motor so that the rotational speed ω is brought to a target value ωT.
The pulse period τP is measured by a count value C of a reference clock CLK having a designated frequency FC. The count value C is given by the following formula, where FP is the frequency of the pulse signal PI.C=FC/FP 
The formula can be rewritten asα=FC/FP2 where α is the weight (C/FP) of the count value C per unit quantity of the frequency FP.
The weight α is the ratio of the count value C to frequency FP when measuring the pulse period τP of the input pulse PI by the count value C. This formula shows that a changes greatly in inverse proportion to the square of FP. For example, a 100-fold change in FP produces a 10,000-fold change in α. Therefore, in the case when performing feedback control on the rotational speed ω of the motor using an error signal Ve, which is proportional to the difference between the present value and the target value of the count value C, when the target rotational speed ωT changes, the generation gain of the error signal Ve changes greatly by the change of the frequency FP that corresponds to ωT, and consequently the gain of the feedback loop also changes greatly.
For example, the speed of a motor that drives a zoom mechanism of a camera changes in accordance with the speed of hand trembling and the movement of the photographed object. At that time, α decreases as the speed of the movement to be tracked becomes higher, and an inconvenience arises in which the trackability of feedback control is reduced.
Thus, the conventional motor speed control based on pulse period τP has an aspect that it is difficult to handle. Here, if the count value C is converted into a signal that corresponds to the frequency FP by calculating the reciprocal of the count value C and multiplying the reciprocal by a suitable factor, then the error signal Ve in feedback control becomes a linear function of the frequency FP. In this case, the gain of the feedback loop is a constant value that does not depend on the target rotational speed ωT.
[Patent Document]
Japanese Laid-open Patent Publication No. 10-174474